Recently, electronic devices for movable communications are required to process a large volume of information at a high speed, and electrical signals which they handle are increasing in frequency. However, intensity of a signal tends to decay faster as its frequency increases. Therefore, the printed-wiring boards in this field need board materials of low transmission loss. In other words, it is necessary to use resin materials of low relative dielectric constant and dielectric loss tangent in a high frequency bandwidth for these boards.
For electronic devices, e.g., computers, high-speed microprocessors working at an operating frequency exceeding 500 MHz have been developed and signal frequency has been increasing, in order to allow them to treat a larger volume of information in a shorter time. One of the problems which have come to the fore in those devices handling high-speed pulse signals is delay on the printed-wiring board. Signal delay time on a printed-wiring board increases in proportion to the square root of relative dielectric constant of the insulator around the wiring. Therefore, the wiring boards for computers or the like need resins of lower relative dielectric constant as the board materials.
The related industries have been using thermoplastic resin materials of low relative dielectric constant and dielectric loss tangent, e.g., fluorine-based ones, to cope with the increased signal frequency. These materials, however, tend to lack fluidity resulting from their high melt viscosity, which causes problems, e.g., need for high temperature and pressure for pressing, and insufficient dimensional stability and adhesion to plated metals. Several proposals have been made to solve these problems, e.g., use of a composition comprising epoxy-based resin and cyanate ester which is known as one of the resins of lowest relative dielectric constant and dielectric loss tangent among thermosetting resins (Japanese Patent Publication No. 46-41112) as a composition of cyanate ester, and a composition comprising bismaleimide, cyanate ester and epoxy-based resin (Japanese Patent Publication No. 52-31279).
Use of thermoplastic resins is also proposed to improve the high-frequency characteristics. These resins include resin compositions based on a polyphenylene ether (PPO or PPE), which shows good dielectric properties among heat-resistant, thermoplastic resins, e.g., a resin composition comprising a polyphenylene ether, crosslinkable polymer and monomer (Japanese Patent Publication No. 5-77705), and another one comprising a polyphenylene ether having a specific settable functional group and crosslinkable monomer (Japanese Patent Publication No. 6-92533).
The other resin compositions proposed to improve the high-frequency characteristics include those comprising a cyanate ester resin and polyphenylene ether having good dielectric properties, e.g., a composition comprising a cyanate ester, bismaleimide and polyphenylene ether (Japanese Patent Laid-open Publication No. 63-33506), and another one comprising a product by the reaction between a phenol-modified resin and cyanate ester, and polyphenylene ether (Japanese Patent Laid-open Publication No. 5-311071). Another resin composition as a heat-resistant forming material of good dielectric properties is comprising a polyphenylene ether and cyanate ester resin kneaded with each other (Japanese Patent Publication No. 61-18937).
On the other hand, printed-wiring boards, not limited to those for signals of higher frequency, have been becoming more and more densified by increasing number of layers for the laminate, making the laminate thinner, and decreasing through-hole size and pitch as electronic devices becoming more compact and more functional. Therefore, the laminate is increasingly required to have higher heat resistance, drill-machinability and insulation characteristics, among others. The methods which have been widely used to improve heat resistance and insulation characteristics of the resins include increasing their glass transition temperature (Tg) to improve the properties of their set products. However, improvement of the resin alone is insufficient to fully satisfy the above characteristics.
One of the methods to solve these problems is use of an inorganic filler as one component for the resin composition. Inorganic fillers have been studied not only as a bulking agent but also as an agent for improving properties of the composition, e.g., dimensional stability and resistance to moisture and heat. More recently, use of a special filler has been studied to provide the composition with excellent functions, e.g., high dielectric constant, low dielectric loss tangent, high heat radiation and high strength.
Under these circumstances, incorporation of an inorganic filler is proposed also for resin materials which can handle high-frequency signals to improve their properties, e.g., heat resistance and dimensional stability. Some of the filler-incorporated resin compositions proposed so far include those comprising a cyanate ester and bismaleimide, and cyanate ester, bismaleimide and epoxy-based resin (Japanese Patent Publication No. 63-33505); polyphenylene ether and crosslinkable monomer (Japanese Patent Laid-open Publication Nos. 62-275744 and 4-91160); and phenol-modified polyphenylene ether and epoxy resin (Japanese Patent Publication No. 10-212336).
However, the method disclosed by Japanese Patent Publication No. 46-41112 or 52-31279, although giving a resin composition of slightly decreased relative dielectric constant, involves a problem of insufficient high-frequency characteristics of the composition, resulting from incorporation of a thermosetting resin other than a cyanate ester resin.
The method disclosed by Japanese Patent Publication No. 5-77705 or 6-92533, although giving a resin composition of improved dielectric properties, involves a problem of high melt viscosity and hence insufficient fluidity of the composition, resulting from polyphenylene ether as the major component, which is inherently thermoplastic polymer. The resin composition, therefore, is unsuitable for laminates because it needs high temperature and pressure in the pressing step, and also unsuitable for multi-layered printed-wiring boards which are treated to fill the groove between fine circuit patterns because of its insufficient formability.
The method disclosed by Japanese Patent Publication No. 63-33506 or 5-311071, although giving a resin composition of slightly improved dielectric properties, involves a problem of still insufficient high-frequency characteristics of the composition, resulting from the thermosetting resin used in combination with the polyphenylene ether, because it is a product of the reaction between the bismaleimide and cyanate ester resin, or between the phenol-modified resin and cyanate ester, and brings the adverse effect(s) of the component other than the cyanate ester. Increasing the polyphenylene ether content to improve high-frequency characteristics of the composition may cause a problem of deteriorated formability resulting from high melt viscosity and hence insufficient fluidity of the composition, as is the case with the above-described polyphenylene ether-based one.
The resin composition comprising a polyphenylene ether and cyanate ester resin kneaded with each other (Japanese Patent Publication No. 61-18937), although having good dielectric properties and relatively good formability, because of decreased melt viscosity resulting from modification with the cyanate ester resin, tends to have the dielectric properties of high relative dielectric constant for its low dielectric loss tangent, when the cyanate ester is separately incorporated as a setting component, with the result that transmission loss may not be sufficiently reduced in a GHz bandwidth. Moreover, decreasing the cyanate ester content to decrease dielectric loss tangent of the composition, which is accompanied by increased polyphenylene ether, may cause a problem of deteriorated formability resulting from high melt viscosity and hence insufficient fluidity of the composition, as is the case with the above-described polyphenylene ether-based one.
In the method which incorporates an inorganic filler in a resin composition to make the laminate of the composition more functional (Japanese Patent Publication No. 63-33505), the filler selected from the common ones begins to settle gradually when incorporated in a varnish. It is therefore necessary to disperse the filler by an adequate procedure, e.g., stirring the composition again, before it is spread. However, it may not be sufficiently dispersed by stirring alone, when it settles massively to agglomerate. The filler may cause other problems in the prepreg production step; it will settle in a portion in a varnish tank or impregnation tank where varnish tends to accumulate, and also will be gradually deposited on a roll or the like, to decrease spreadability (workability) of the composition, significantly deteriorating outer appearances of the prepreg and preventing uniform dispersion of the filler, and hence deteriorating properties of the laminate of the composition, e.g., adhesion at the interface, resistance to moisture, drill-machinability and insulation characteristics.
One of the methods to improve dispersibility of the filler is coating the filler particles beforehand with a coupling agent or the like. However, the surface treatment increases the filler cost and greatly limits types of the commercial available products, and it is difficult to select the treated filler suitable for a variety of resin composition production systems. On the other hand, quantity of a filler incorporated in resin materials tends to increase, for improving their functions more, which is accompanied by significantly increased quantity of the filler settling in a system and deposited on a roll or the like. Therefore, the filler has been increasingly required to be more dispersible and thixotropic. The conventional treatment with a coupling agent is difficult to satisfy these characteristics.
When a filler is to be surface-treated, it is normally dried under heating after being treated, e.g., by being immersed in, or sprayed with, a diluted solution of the treatment agent. The drying step involves two types of problems, oligomerization of the coupling agent on the treated filler surface to form a physically adsorbed layer, and agglomeration of the filler particles, which requires finely crushing the agglomerates before the filler is incorporated in a varnish, which, in turn, causes a problem of leaving an unevenly treated layer on the filler surface. The physically adsorbed layer and unevenly treated layer, when formed, deteriorate adhesion of the resultant laminate at the interface between the filler and resin.
One method directly adds a coupling agent while a varnish is being incorporated (Japanese Patent Laid-open Publication No. 61-272243). The varnish used in this method is viscous, because the resin is incorporated beforehand. Therefore, it can avoid agglomeration of the filler particles to some extent, but is difficult to selectively direct the coupling agent evenly onto the filler particle surfaces, causing problems of insufficient adhesion at the interface between the inorganic filler and resin, and insufficient dispersibility of the filler in the resin.
Particularly, incorporation of an inorganic filler in the polyphenylene ether-based resin material, disclosed in Japanese Patent Laid-open Publication No. 62-275744, 4-91160 or 10-212336, involves a problem of very high viscosity of the molten polyphenylene ether and of the solution of the ether dissolved in a solvent, making it difficult to evenly disperse the filler in the resin. This significantly agglomerates the filler particles, producing the defects, e.g., voids, at the interface between the inorganic filler and resin, and deteriorates properties of the set product and laminate of the composition, e.g., resistance to moisture, drill-machinability and insulation characteristics.
Surface treatment of an inorganic filler on a commercial scale is completed in a very short time, even when the filler is treated with a common, commercial coupling agent. As a result, the filler particles are surface-treated insufficiently, because they are covered only with a rigid, thin layer unevenly. Moreover, the physically adsorbed layer tends to be eluted out into the resin layer, and elution of the adsorbed layer, when occurs, is likely to cause problems, e.g., unevenly set resin in the vicinity of the interface, and adhesion to the interface between the filler and resin, resulting from reduced strength. As discussed above, it is difficult to disperse an inorganic filler in a highly viscous polymer, e.g., polyphenylene ether, without agglomerating the filler particles. Therefore, incorporation of an inorganic filler in the resin has caused problems of deteriorated properties of the laminate of the resultant composition, e.g., resistance to moisture, drill-machinability and insulation characteristics, as discussed above. Moreover, a resin material based on a thermoplastic resin, e.g., polyphenylene ether, involves a problem of insufficient dimensional stability and adhesion to plated metals.
The present invention has been developed under these situations. It is an object of the present invention to provide a resin composition exhibiting excellent dielectric properties in a high-frequency bandwidth, as formable and machinable as a laminate of the conventional thermosetting resin, e.g., epoxy resin, and capable of giving laminates and printed-wiring boards of high heat resistance and excellent reliability of electrical insulation. It is another object of the present invention to provide a process for producing a varnish, prepreg and metal-clad laminate using the above resin composition, and the resin composition itself.